General overview »
Magnetic Nanoparticles »
Standardization »
Characterization and
analysis methods »
DC magnetization and AC
susceptometer analysis »
Medium and high frequency
AC susceptometry »
Mössbauer spectroscopy »
Electron microscopy »
XRD and SAXS »
SANS »
Electron microscopy »
Ferromagnetic resonance »
Dynamic light scattering and
electrophoretic light scattering »
Field-flow fractionation »
Magnetic modelling »
Magnetorelaxometry »
Magnetic particle spectroscopy »
Magnetic particle rotation »
Magnetic separation »
NMR R1 and R2 relaxivities »
Magnetic nanoparticle bio-detection »
Magnetic hyperthermia measurements »
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Medium and high frequency AC susceptometry
Dynamic magnetic measurements (or AC susceptometry) give information of the relaxation properties of the nanoparticles. The relaxation of magnetic nanoparticles can take place via the Brownian or the internal Néel mechanism. If the nanoparticles are dispersed in a liquid medium, such as DI water, both relaxation mechanisms are present while the faster of the two dominates. If the particles are immobilized, e.g., by freeze-drying, or in the form of powder, the Brownian process is inhibited so that the dynamics are determined by the Néel process. Whereas the Brownian time constant is directly proportional to the viscosity of the medium and the hydrodynamic volume of the particles, the Néel relaxation time is solely determined by the volume of the magnetic core multiplied by the anisotropy constant. Consequently, hydrodynamic and core sizes of the nanoparticles can be determined from measurements of the AC susceptibility.
In these AC susceptometry measurements an alternating magnetic field is applied and the in and out-phase of the AC susceptibility is determined. For small AC field amplitudes, susceptibility spectra are generally modelled using the Debye model which can be modified to account for distributions of core and hydrodynamic sizes. AC susceptibility setups for measurements on magnetic nanoparticles cover a very wide range of excitation frequencies, ranging from a few Hz to several MHz, corresponding to time constants over about 7 orders of magnitude. Superposition of a static background field, AC susceptibility measurements can be utilized for studying the magnetic-field-dependence of Brownian and Néel time constants. The magnetic losses can be determined from the out-of phase component and together with the magnetization versus field dependency the specific power absorption in magnetic hyperthermia application can be determined at any field amplitudes (not just the field range where the magnetization is linear to the field).
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